Ligand-Functionalized Adsorbents for the Extraction and Recovery of Rare Earth Elements

2018-10-17T16:28:11Z (GMT) by Jonathan Callura
Rare earth elements (REE) are central components of many developing technologies, but uncertainties surrounding their supply chain have led to concerns about their long-term availability. Recent research has identified new potential sources of REE, including brines and other saline waters, that are abundant though contain high concentrations of other, less valuable, metals which are prohibitively difficult to separate using currently available methods. The development of new, more selective methods for REE recovery from complex aqueous mixtures represents a significant technical challenge and a large potential opportunity to close the growing supply/demand gap for critical materials.<br>The overall goal of this research was to develop ligand-functionalized adsorbents and evaluate their potential as a method for REE extraction from saline waters. This was accomplished through the following research objectives: (1) Investigate adsorption of REE onto functionalized model supports; (2) Characterize large-scale functionalized polymer resins and evaluate their REE binding mechanisms; and (3) Determine selectivity and study fixed-bed column performance of functionalized adsorbents.<br>The research related to Objective 1 addressed critical research gaps by studying the performance of silica adsorbents functionalized with three ligands – diethylenetriaminepentaacetic dianhydride (DTPADA) phosphonoacetic acid (PAA), N,N-bis(phosphonomethyl)glycine (BPG) – in a range of application-relevant conditions. Attachment of ligands to the surfaces was completed by connection to surface amines using either a ‘Bottom-Up’ (BU-) or ‘Top-Down’ (TD-) approach. ‘Bottom-Up’ materials were synthesized by attaching ligands to pre-silanized silica surfaces, while ‘Top-Down’ materials were made by first combining ligands with aminosilane groups and then attaching the ligand-silane structures to raw silica surfaces. Amine conversion efficiencies were 9-28% for the standard ‘Bottom-Up’ synthesis method, yielding ligand concentrations of 0.268 mmol/g (BU-DTPADA), 0.15 mmol/g (PAA), and 0.086 mmol/g (BPG). The ‘Top-Down’ approach yielded improved ligand attachment of 0.463 mmol/g (TD-DTPADA) and better performance in general for the DTPADA ligand. Optimal pH conditions for REE uptake in brine solutions were found for DTPADA- (pH 2.5), PAA- (pH 7), and BPG-functionalized silica (pH 1-3 or >5). The REE adsorption performance of the functionalized adsorbents was largely unimpeded by the presence of competing ions (Ca, Mg, Zn, Fe, Al). Tests with real brines (I ∼ 3 M) showed >90% efficiency in REE recovery, which improved at higher temperatures (up to 100 °C). Effective elution of REE was accomplished with 0.7 N HNO3, and performance of the adsorbents improved with additional usage cycles. The improved performance for cycles two and three was attributed to the washing away of physically deposited ligands (i.e., non-covalently bound) left over from the functionalization process.<br>This work was advanced by using the same ‘Bottom-Up’ synthesis method to generate new, large-scale functionalized polymer resins in work performed in relation to Objective 2. Large-scale functionalized adsorbents allow the use of continuous-flow extraction processes, such as fixed-bed adsorption columns. Before continuous-flow systems can be designed, it is necessary to study their properties and performance in batch conditions to gain insight into their adsorption capabilities. Polymeric resin beads, composed of polystyrene-divynlbenzene copolymers containing primary amine surface groups, were functionalized with DTPADA, PAA, and BPG. Their physical and chemical properties (surface area, pore volumes, ligand concentrations, and acidity constants) were investigated, and the REE binding mechanisms were probed using batch experiments and complexation models. Amine concentrations were measured on the non-functionalized supports (4.0 mmol/g) and ligand grafting efficiencies were found for DTPADA- (0.42 mmol/g; 11% amine conversion), PAA- (0.33 mmol/g; 8% amine conversion), and BPG-functionalized resins (0.22 mmol/g; 6% amine conversion). Kinetic studies revealed that the functionalized resins generally followed pseudo-second order binding kinetics with intraparticle diffusion serving as a rate-limiting step. Capacity estimates for total REE adsorption based on Langmuir qMax were found to be 0.12 mg/g (amine), 0.72 mg/g (PAA), 3.0 mg/g (BPG), and 2.9 mg/g (DTPADA). Addition of ligands to the resin surfaces greatly improved REE adsorption in all cases. Adsorption isotherm results suggest that the observed REE uptake above Langmuir qMax values may be the result of non-selective multilayer physisorption by other chemical groups on the resin surfaces rather than selective chelation by surface-tethered ligands.<br>The gap between research and practical application was bridged through the research under Objective 3. This portion of the project included the evaluation of resin selectivity and systematic study of mass transport properties in a fixed-bed column system, while determining the parameters required to design and scale up systems. Separation factors were determined for BPG-functionalized large-scale polymer resin beads and non-functionalized aminated resins by conducting multi-element competitive adsorption isotherm experiments, and the BPG-functionalized resins were found to be highly selective for REE, with separation factors for REE over other metals as much as 700 times higher than the non-functionalized aminated resins. Resin performance was measured in fixed-bed adsorption columns and the breakthrough of REE from the column packed with BPG-functionalized resins was influenced by influent concentration and flow rate. Mass transfer zones in the columns were 95% shorter and moved 13% faster with influent REE concentrations of 10 mg/L compared to 0.1 mg/L, leading to more rapid column exhaustion at higher concentrations. The influence of column geometry on REE breakthrough was studied and superficial fluid velocities of 0.4 m/hour were found to be suitable for operation. Reusability of the BPG-functionalized resins was studied through batch and column elution and reuse tests, and performance was found to improve after the first 1-2 cycles and remained stable thereafter. Ligand concentrations were measured on the BPG-functionalized resins and were found to be stable for at least six use cycles after an initial HNO3 wash. The data and interpretations offered in this thesis have advanced the understanding of performance characteristics and surface properties of ligand-functionalized adsorbents for REE extraction from saline waters. The materials described and studied in this work have potential for scalable resource recovery schemes due to their selectivity and improved REE binding capacity compared to conventional materials. Outcomes from this research will help advance the development of adsorption systems tailored to a variety of potential feedstocks which allow the recycling and recovery of valuable REE components. <br>